1. Field of the Invention
This invention pertains to heat transfer, and more particularly to apparatus for transferring heat in recirculatory fluid systems.
2. Description of the Prior Art
Various equipment and methods have been developed to raise the temperature of a fluid inside an enclosure and to employ the heated fluid for useful purposes outside the enclosure. Important applications of such equipment and methods include recirculatory systems. In a recirculatory system, a recirculating fluid flows in a generally closed loop through a first enclosure where it is heated, through a second enclosure where the heated fluid is utilized, and back to the first enclosure for reheating. Typically, the recirculating fluid is heated by a heat exchanger that burns oil or gas in a combustion chamber. Elongated ducts conduct the hot products of combustion from the combustion chamber to a flue. The fluid flows past the heat exchanger ducts and combustion chamber to be heated by the hot products of combustion therein. Well known examples of recirculatory fluid systems include ovens, dryers, and climate control systems.
In many recirculatory fluid systems, recirculating air is heated by a heat exchanger that is located inside a relatively small pocket that forms a part of the system. The heated air flows through the pocket to a much larger working chamber in which the hot air heats, dries, or otherwise affects objects or persons in the working chamber. The cooled recirculating air then flows back to the pocket for reheating.
For several reasons, prior heat exchangers used in many recirculatory fluid systems have not been completely successful. One reason for unsatisfactory performance is that the space available for the heat exchanger is usually quite limited. Prior heat exchangers that could fit into the available pocket space often lacked the capacity to transfer the requisite heat to the recirculating air. To obtain the necessary performance from prior heat exchangers, it was frequently necessary to mechanically induce the hot products of combustion to flow from the combustion chamber to the flue. For that purpose, an inducer in the form of a fan or blower was often installed in the stream of the products of combustion downstream from the combustion chamber. Although the inducers did increase the flow rate of the products of combustion, a serious disadvantage was that the inducer adversely affected the combustion of the fuel. Particularly, the inducer tended to destabilize the burner flame in the combustion chamber and therefore cause a decrease in burner efficiency.
To suit the limited space available, prior heat exchangers were designed unsymmetrically about their combustion chambers. In addition, the ducts were arranged to provide a series path for the flow of the products of combustion from the combustion chamber to the flue. Because of the unsymmetrical and series flow design, the combustion chamber and the various ducts occupied respective portions of the pocket such that recirculating air flowing past the ducts did not flow uniformly past the combustion chamber, and air flowing past the combustion chamber did not flow uniformly past the ducts. The recirculating air that flowed past the combustion chamber was exhausted from the pocket at a higher temperature than the temperature of the air that flowed past the ducts. Similarly, the air that flowed past the ducts adjacent the combustion chamber was exhausted from the pocket at a higher temperature than the air that flowed past the ducts adjacent the flue. The result was that the stream of recirculating air leaving the pocket had a distinct temperature gradient across its cross section. The temperature gradient was not dissipated by continued downstream flowing of the air into the working chamber, that is, the various temperature layers in the stream of air did not mix to form a stream having a uniform temperature. The result was undesirable temperature differences within the working chamber, because some locations therein were hotter than desired and some locations were cooler than desired.
A related disadvantage of the prior heat exchangers in many recirculatory fluid systems was that the recirculating air that flowed past the combustion chambers did not do so effectively. Certain areas of the combustion chambers were not properly scrubbed by the recirculating air, with the result that hot spots were created on the combustion chamber. Those hot spots produced undesirable thermal stresses in the combustion chamber. Further, heat that could have been transferred to the air was wasted instead.
A corollary problem of the space limitations for heat exchangers in recirculatory fluid systems is that the heat exchanger filled a very large percentage of the pocket space. As a result, the flow of recirculating air past the heat exchanger was excessively restricted. Consequently, less than the optimum air flow could occur without the use of an undesirably large fan or blower.
Thus, a need exists for improved heat exchangers for use in confined enclosures.